The present invention relates to optical scanning systems which provide rapid switching between two viewing modes (for example, between narrow and wide fields of view), and, more particularly, to those scanning systems used in conjunction with Forward Looking Infrared ("FLIR") imaging devices.
The requirement for high performance, low altitude, single piloted aircraft missions has imposed severe demands upon the design of systems for the acquisition and utilization of imagery, particularly infrared or reconnaissance imagery. Typically, such low altitute aircraft sensors must simultaneously provide video information from two different fields of view; a wide field of view for general pilotage and navigation, and a narrow field of view for target screening and tracking. Although it would be possible to perform both viewing functions by providing two separate sensors, this would require a heavier, larger and much more expensive system. As a result, these constraints on sensor size, volume and detector complexity have resulted in a need for single optical imaging systems which can perform these multiple functions.
In such single optical imaging systems, since neither viewing function requires continuous update, it is possible to time share a common detector array between viewing modes, thereby reducing system complexity and cost. However, to time-share the detectors by means of directing video frames covered by the detectors from one function to the other in a selected sequence, it is necessary to switch the optical axis from one field of view to the other during a small fraction of a frame period. At the same time, it is necessary to scan so that a restricted number of detectors in an ordered array can cover the full field of view in a frame period.
In the past, a single sensor providing both scanning and switching functions has required a separate assembly for each function. For example, in some systems the scanning function has been performed by oscillating a plane mirror, or rotating an external multifaceted mirror or refractive wedges. The function of switching between different optical systems or fields of view has been provided by either a second oscillating plane mirror, a light modulator or a mechanical shutter.
Systems incorporating these types of switch and scan assemblies have numerous disadvantages. First, the use of an oscillating mirror for switching fields of view require a large drive impulse and a relatively long settling time relative to the duration of the frame period. A light modulator switch is undesirable since it requires increased aperture and/or component size. The use of oscillating mirrors or rotating refractive wedges for scanning is generally undesirable since they are limited in field angle, and are nonlinear at wider fields of view. An externally faceted drum type scanning mirror generally requires increased system volume, and is, therefore, less desirable for many applications.
It is, accordingly, a primary object of the present invention to provide an improved imaging sensor having a single scanning and field switching assembly. It is a further object of the present invention to provide a sensor which is both compact and inexpensive, and which, by virtue of its scan mechanism and shared detector electronics, provides periodic frame update for two or more fields of view. A further object of the present invention is to provide a sensor switch-while-scan mechanism which is lightweight, so that in combination with a low angular velocity, it has low inertial force, and therefore a high degree of overall stability.